The effects of charging ratio on the thermal performance of a thin novel flat heat pipe (FHP) heat spreader under natural air convection were investigated systematically. Temperature difference and temperature distribution along the central axis of the flat heat pipe were measured carefully. Results show that there exists an optimal charging range for the FHP and a very small temperature difference was achieved, demonstrating superior heat spreading capability. Furthermore, a temperature inverse increase was observed in the condensation section. In addition, within the range of optimal charging ratios the flat heat pipe was insensitive to the gravity force. All the results indicate this novel type of two-phase heat spreader is very suitable for mobile electronics cooling in view of its high-performance as well as its easy processing. (C) 2016 Elsevier Ltd. All rights reserved.